Something sour-ish, maybe a little bit pungent to add a bit of oomph to the almost buttery smoothness of the sour cream, most likely. Maybe a touch of mustard? I might be tempted to add just a wee bit of vinegar to it. I might even suggest tarragon on the flimsy basis that my favorite potato salad which involves dill has tarragon in it too. :D

(Hmm. Will have to try it. :D)